The present invention relates to expandable endoprosthetic devices, generally called stents, which are implanted in a patient's body lumen, such as blood vessel, to maintain the patency thereof. More particularly, the present invention relates to a triangular shape strut pattern for increased coverage and reduced gap size between the struts.
Stents are generally tubular-shaped devices which function to hold open a segment of a diseased blood vessel or other anatomical lumen. For example, stents are useful in the treatment and repair of blood vessels after a stenosis has been compressed by a procedure such as percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or when the stenosis is removed by atherectomy or other means. They are also suitable for use to support and scaffold a dissected arterial lining that might occlude the fluid passageway or to repair an injured vessel wall as in an aneurysm.
Various means are known in the art to deliver and implant stents. One method frequently described for delivering a stent to a desired intraluminal location involves mounting an expandable stent on an expandable member such as a balloon provided on the distal end of an intravascular catheter, advancing the catheter to the desired location within a patient's body lumen, inflating the balloon on the catheter to expand the stent into a permanently expanded condition, deflating the balloon, and withdrawing the catheter.
As stents are normally employed to hold open an otherwise constricted or occluded lumen, a stent must exhibit sufficient radial or hoop strength in its expanded state to effectively counter the anticipated closing forces. Not only is it advantageous to distribute such loads over as much of the stent as possible, but it is most beneficial to distribute the load over as much of the lumen wall as possible. This helps minimize injury to the vessel wall. Also by minimizing the gaps between stent struts, it is possible to prevent prolapse of the plaque or the vessel wall in the open areas between the struts into the lumen. As a consequence, it is desirable to maximize the coverage of the lumen wall by creating small, uniform openings between the stent struts. It is, however, simultaneously necessary for the stent to be as small and compact as possible in its collapsed state in order to facilitate its advancement through the lumen to the delivery site. As large an expansion ratio as possible is therefore most desirable.
A number of very different approaches have been devised in an effort to address these various requirements. One approach calls for the stent to be constructed entirely of wire. The wire is bent, woven, or coiled to define a generally cylindrical structure in a configuration that has the ability to undergo radial expansion. The use of wire, however, has a number of disadvantages. For example, a substantially constant cross-sectional area of a wire might cause greater or lesser than an ideal amount of material to be concentrated at certain locations along the stent. Additionally, wire has limitations with respect to the shapes it can be formed into thus impairing the expansion ratio, coverage area, and strength that can ultimately be attained. Regarding strength, some have welded sections of wire together to increase strength albeit with a substantial increase in manufacturing costs and possible materials problems caused by welding heat.
As an alternative to wire-based structures, stents have been constructed from tube stock. By selectively removing material from such tubular starting material, a desired degree of flexibility and expandability can be imparted to the structure. Chemical etching techniques as well as laser-cutting processes are utilized to remove material from the tube. Laser cutting provides for a high degree of precision and accuracy through which very intricate shapes of material can be removed leaving behind a complex strut pattern.
The performance parameters of a stent is often a function of the pattern in which material is removed from the tube stock. The selection of a particular pattern sometimes has a profound effect on the coverage area, expansion ratio, and strength of the resulting stent.
As shown in FIGS. 1–3, prior art stent designs have attempted to increase vessel coverage while maintaining expansion ratio and strength. For instance, the stent design as shown in FIG. 1 illustrates a stent 1 having bridging elements 2. Here, the bridging element 2 is widened to increase the surface area 3 in order to achieve a commensurate increase in vessel coverage. Yet another prior art stent 40, as shown in FIG. 2, has a strut pattern shown in the compressed state that increases vessel coverage by increasing the density of the struts 5 forming the stent body. The effectiveness of this strut pattern lies in the ability to “nest” the strut pattern. That is, the peaks 7 of one ring fit into the interstices of an adjacent ring and the valleys 8 of the adjacent ring fit into the interstices of the first ring so that two adjacent rings essentially overlap. The struts 5 are also curved to pack closely to the bridge member 6.
In FIG. 3, the prior art strut pattern is identical to that shown in FIG. 2, but is depicted in the fully expanded state. The nested struts have moved away from each other without structures interfering in that motion. The prior art stent designs thus provide improved vessel coverage without dramatically increasing the quantity of material used. However, there is room for further improvements.
Additionally, one of the continuing goals of endovascular therapy includes providing stenting solutions to smaller and smaller vessels more distant along the patient's vasculature away from the physician. To this end, prior art stents have been designed and developed to exhibit higher degrees of compressibility to achieve a smaller compressed delivery profile capable of fulfilling the goal of delivery to distant vessels. One approach to stent design capable of achieving a smaller compressed stent profile is through reducing the mass to space ratio of the stent body. Having less mass and high expandability, the stent may be compressed into a highly compact profile capable of achieving the goal of delivery to smaller and more distant vessels.
Yet a continuing obstacle encountered with prior art designs attempting to reduce the mass to space ratio is that as the mass to space ratio of the stent decreases, so does the vessel coverage provided by the same stent. As can be seen from the foregoing discussion, the goal of decreasing the compressed profile of the stent by reducing the amount of structural material is somewhat antithetical to the goal of providing increased vessel coverage by using more stent material to increase the stent surface area.
Therefore, what has been needed and heretofore unavailable is a highly compressible stent for delivery into highly tortuous and distant vessels, yet still is able to provide acceptable coverage of the target vessel in order to decrease the possibility of vessel wall or plaque prolapse. The present invention satisfies these needs.